This invention relates to combining plural laser beams coherently, and most particularly to devices using the Talbot effect for coherent combining.
Linear arrays of lasers have been developed that produce higher power by combining many individual lasers. Monolithic surface emitting geometries allow laser arrays to be constructed in two dimensions as well. In addition to high power, however, many applications also require a source with high brightness, or power per unit area per unit solid angle. High brightness requires mutual coherence among the individual lasers. In addition, the individual beams must be combined to produce a single-lobed, far-field pattern with negligible side lobes.
One method of meeting these criteria is by use of the Talbot effect. The Talbot effect refers to the self-imaging properties of periodic structures which can be used to establish coherence across an array of semiconductor lasers. The Talbot effect, using diffractive coupling, produces an image without the aid of lenses, and makes possible a very compact external cavity for coherent beam addition. Diffractive coupling is used to establish mutual coherence across a laser array. The light from a single lasing aperture diffracts into adjacent apertures to provide the necessary coupling. This technique allows the lasers to be separated by large distances, making heat removal easier, and applies to both one- and two-dimensional laser arrays. An apparatus using this technique is disclosed in Leger et al., U.S. Pat. No. 4,813,762, which is incorporated herein by reference.
Also known is that a coherent laser array is capable of lasing in at least two different lateral modes of operation: fundamental mode and highest-order mode. Arrays lasing in highest-order mode produce double-lobed far-field diffraction patterns, while those lasing in fundamental mode produce a single-lobed pattern. FIG. 5 demonstrates an example of a single-lobed far-field pattern associated with a laser operating in fundamental mode. The single lobe demonstrates a concentration of lasing power on-axis. Contrasted with this is FIG. 11, which demonstrates a double-lobed pattern. Rather than concentrating lasing power to a single on-axis lobe, the power is split into two separate lobes. Since most applications require a single intense spot in the far field, it is preferred that a high-power laser array lase in fundamental mode.